1. Field of the Invention
The present invention relates to a thermal fixing device and an image forming apparatus including the thermal fixing device.
2. Description of the Related Art
An image forming apparatus such as a laser printer is generally provided with a thermal fixing device including a heat roller and a pressure roller. The image forming apparatus thermally fixes a toner transferred on a sheet in a period when the sheet passes through between the heat roller and the pressure roller.
In the thermal fixing device, there is known a device in which in order to prevent a sheet from being wound around a heat roller, a peeling pawl is provided at a downstream side of the heat roller in a rotation direction with respect to a contact portion between the heat roller and a pressure roller. In the thermal fixing device as stated above, the sheet after fixation is peeled off from the heat roller by the peeling pawl.
However, in a case where the peeling pawl is constantly made to be in contact with the heat roller during the fixing operation, that is, during the rotation of the heat roller, toner is deposited on the peeling pawl, the deposited toner is again adhered to the heat roller, and smudges are created on the sheet to be fixed, or the heat roller is worn down by the continuous contact with the rotating heat roller, and the durability of the heat roller is lowered.
In order to solve the problem above, there is proposed a configuration as disclosed in JP-A-8-054801 that a peeling pawl is axially supported to be capable of coming in contact with and separating from a heat roller, and an air pressure receiving part of the peeling pawl is exposed to the air blown from an air pump so that the peeling pawl is brought into contact with the heat roller, and a contact/separation operation is performed by ON/OFF of the air pump by a solenoid so that the peeling pawl comes in contact with the heat roller only in a period necessary for peeling off the sheet from the heat roller, that is, only for a length of about 10 mm from the front end of the sheet.
However, in the configuration disclosed in the JP-A-8-054801, it is necessary to provide a large expensive mechanism, such as the air pump for blowing the air, the solenoid for turning On/OFF the air pump, or a blowing fan.